1. Field of the Invention
This invention relates generally to audio systems, and, more particularly, to a low distortion audio muting circuit for an audio system.
2. Description of the Related Art
Today, audio systems typically come equipped with a muting circuit to disable the audio output when the signal is undesirable or unnecessary. The audio systems typically include audio amplifiers that amplify the input to drive a load such as a speaker. When muting is activated, the amplified audio signal is heavily attenuated with respect to the input signal.
It is desirable to have a muting circuit that not only functions effectively while an audio system is powered-on but also during the system's power on/off cycle. This is because, when a power source of an audio amplifier is turned on or off, a large pop noise is emitted from the speakers. This discordant noise is caused by a sudden surge of voltage during the power on/off cycle of the audio amplifier. Even though the pop occurs momentarily, it can cause damage to sensitive audio speakers.
One attempt that the prior art has made to fix the power on/off pop is to use an NPN bipolar junction transistor (referred to as NPN transistor hereinafter) to ground the audio output signal as the power supplies rise, thereby muting the signal. FIG. 1 illustrates a circuit 1 that mutes an output signal V.sub.out 25 of an audio amplifying circuit 2. As shown, the muting circuit 1 includes an NPN transistor 15, a mute controller V.sub.cntl 20, and a resistor 50, while the audio amplifying circuit 2 includes an operational amplifier 10, an input source V.sub.in 5, a capacitor 55, and resistors 35, 40, 45.
The input source V.sub.in 5 is connected between a ground node 30 and a first terminal of the input resistor 35. The second terminal of the input resistor 35 is coupled to the non-inverting input of the operational amplifier 10. The output of the operational amplifier 10 is coupled to a first terminal of the capacitor 55 and a first terminal of resistor 45. A second terminal of the capacitor 55 is connected to the collector terminal of the NPN transistor 15. A second terminal of the resistor 45 is coupled to the inverting input of the operational amplifier 10 and a first terminal of the second feedback resistor 40. A second terminal of the second feedback resistor 40, the second terminal of the mute controller V.sub.cntl 20, and an emitter terminal of the NPN transistor 15 are coupled to the ground node 30. The resistor 50 is coupled between a first terminal of the mute controller V.sub.cntl 20 and a base terminal of the NPN transistor 15.
While the muting circuit 1 of FIG. 1 is generally able to ground the audio output signal V.sub.out 25, it still suffers from several shortcomings: it only marginally mutes the power on/off audio pop; it inadequately mutes the audio output signal V.sub.out 25; it introduces distortion; and, it is not suitable for muting multiple audio output signals V.sub.out 25.
The muting circuit 1 is only marginally capable of muting the power on/off pop because the NPN transistor 15 must first be turned on before it can conduct current to the ground, thereby shorting the output to ground and clamping (i.e., muting) the audio output signal V.sub.out 25. To turn on the NPN transistor 15, the mute controller V.sub.cntl 20 must supply at least one to two volts. But because the mute controller V.sub.cntl 20 is unable to provide the necessary voltage until the power supply (not shown) reaches at least one to two volts, the NPN transistor 15 remains off. Thus, the NPN transistor 15 is unable to ground the output signal V.sub.out 25 and prevent the audio pop, until it is turned on.
The muting circuit 1 of FIG. 1 is unable to adequately mute the audio output signal V.sub.out 25 because the NPN transistor 15 cannot properly clamp the negative cycle of the audio output signal V.sub.out 25. FIG. 2 shows the input signal generated by the input source V.sub.in 5 and the resulting audio output signal V.sub.out 25 while the NPN transistor 15 is on. As illustrated, the audio output V.sub.out signal 25 is muted during the positive cycles, but not the negative cycles. The inability of the NPN transistor 15 to properly clamp the negative cycles of the audio output signal V.sub.out 25 results in unwanted noise, which can be audible through the speakers.
In addition to the inadequacies of properly muting the audio output signal V.sub.out 25, the prior art muting circuit 1 distorts the negative half of the audio output signal V.sub.out 25, even when deactivated (i.e., unmuted). During the negative cycle of the audio output signal V.sub.out 25, the NPN transistor 15 becomes forward biased across its collector-base junction because the voltage at the collector terminal falls below the voltage at the base terminal of the NPN transistor 15. When the NPN transistor 15 becomes forward biased across its collector-base junction, the current flows from the base to the collector terminal and into the output terminal of the operational amplifier 10, as well as to the load (not shown). The additional current flow into the output of the operational amplifier 10 and into the load distorts the audio output signal V.sub.out 25 by up to ten percent. Generally, a distortion of approximately 0.1 percent can be audible.
Another drawback of the muting circuit 1 is that it is not suitable for muting a large number of audio output signals V.sub.out 25. This limitation is best understood with reference to FIG. 3, which depicts the mute circuit 1 of FIG. 1, adapted for muting multiple audio output signals V.sub.out 25, 245. In addition, FIG. 3 also shows a schematic diagram of the mute controller V.sub.cntl 20, which includes a muting control input 200, an open collector buffer 205, a resistor 210 coupled between an output of the open collector buffer 205 and a twelve volt voltage source (not shown).
The mute controller V.sub.cntl 20 controls the NPN transistor 15 of the muting circuit 1. The NPN transistor 15 remains off during periods in which essentially no voltage is applied to the muting control input 200, and turns on when a voltage of approximately five volts is applied to the muting control input 200. When the NPN transistor 15 is on, it mutes the audio output signal V.sub.out 25 by grounding it.
The mute controller V.sub.cntl 20 is capable of muting multiple audio output signals V.sub.out 25 as well. As shown in FIG. 3, first terminals of the resistors 50, 230 are coupled to the output node 250 of the mute controller V.sub.cntl 20. Second terminals of the resistors 50, 230 are coupled to base terminals of the NPN transistors 15, 240, respectively. The audio output signals V.sub.out 25, 245 are coupled to the collector terminals of the respective NPN transistors 15, 240, which, when activated, ground the audio output signals V.sub.out 25, 245. As illustrated in FIG. 3, it is possible to mute additional audio output signals by utilizing additional NPN transistors for each audio output.
In systems having multiple audio outputs, it is often desirable to have the ability to quickly mute all of the audio output signals. One way to quickly mute multiple audio output signals V.sub.out 25, 245 is to utilize only one mute controller V.sub.cntl 20 to drive the multiple NPN transistors 15, 240 to ground the audio output signals V.sub.out 25, 245. When the NPN transistors 15, 240 are on (i.e., during the mute mode), they draw current from the mute controller V.sub.cntl 20, which is capable of providing only enough current to reliably turn on about two NPN transistors 15, 240. Thus, when additional NPN transistors are utilized to mute additional audio output signals, the mute controller V.sub.cntl 20 is unable to fully turn on all of the NPN transistors because of the excessive load. While it is possible to increase the drive capability of the mute controller V.sub.cntl 20 by decreasing the value of the resistor 210, it can be done only to a limited extent due to the increased power dissipation and the inability of the open collector buffer 205 to sink the increased drive capability when unmuted. Another method of increasing the drive capability is to utilize additional mute controllers V.sub.cntl 20 to drive the NPN transistors. But this would require more board space, result in higher power dissipation, and make it difficult to mute all audio output signals quickly.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.